Effect of Paramagnetic Mineral Content and Distribution on Surface Relaxivity in Organic-Rich Niobrara and Haynesville Shales

Abstract

Surface relaxivity (SR) is required to convert nuclear magnetic resonance (NMR) relaxation times to pore size distributions (PSD).Current methodology uses a constant value of SR for an entire well, formation, or rock type, regardless of compositional and textural variations. This approach might result in significant errors in PSD calculation in the presence of paramagnetic minerals that affect SR values. We present SR calculations for Niobrara and Haynesville samples calculated from measurements of surface to volume ratio (SVR) and from measurements of the average pore radius. We measured the transverse relaxation time (T2) with a low field (2 MHz) NMR instrument. We also measured porosity and PSD using nitrogen adsorption (N2). The total specific surface area (TSSA) was measured using N2 and cation exchange capacity (CEC) equivalent surface area. We find that paramagnetic impurities, chlorite and illite in the Haynesville and illite-smectite in the Niobrara, dominate the NMR response and the calculated SR. There is a linear relation between the paramagnetic clay content and SR. The value of SR depends on the calculation method and on the measurement technique for TSSA and TPV. Presence of smectite increased the uncertainty in TSSA and TPV and consequently SR calculations in Niobrara. This uncertainty is lower for high maturity (gas window) Haynesville samples since smectite is absent in these samples. Our SR clay correlations can be used to calculate SR from mineralogy and invert NMR logs and laboratory NMR data to PSD.